Antennas comprising co-linearly arranged half-wave dipoles are widely used for reception and transmission of electromagnetic energy. Such antennas are commonly used in a variety of applications including marine communications. The typical use of the antenna involves the radiation or reception of a signal of sinusoidally varying electromagnetic waves, which travel at the speed of light with a particular frequency and wavelength. The wavelength of the signal determines the most desirable length for an antenna, as is very well-known in the art. As used herein, the term "wavelength" means the wavelength of an intended signal and "half-wave" and "quarter-wave" mean the length of one-half and one-quarter of a wavelength, respectively.
Typically, co-linear stacked dipole antennas comprise one or more sections which house two or more half-wave dipoles or conductor elements which are stacked vertically end to end to form the resulting antenna which may be considerably longer than the wave length employed. The length of each element and the number of elements may be selected to achieve a desired radiation pattern and gain. Generally, each section includes a support tube and a length of conventional coaxial cable which has a center conductor and a braided outer conductor, the two conductors being electrically insulated from one another. The coaxial cables of adjacent elements are cross-fed. That is, the center conductor of a first element is electrically connected with the braided outer conductor of a second element; the braided outer conductor of the second element is electrically connected with the center conductor of a third element, and so forth. Thus, the phase of the current through each element is in phase with that of the next element, creating the desired current phasing.
The antenna design described above has several significant drawbacks, especially in marine and other transportation-related applications. Oftentimes, it is necessary for an antenna of this nature to be relatively long in order to achieve a desired gain. In marine applications, an antenna may be as long as 25 feet. While this length may be acceptable on open waters, it may be inconvenient or dangerous to have an antenna of this size mounted when a boat is passing under a bridge or navigating congested waters. Furthermore, it is desirable to be able to stow the antenna when the boat is in storage or in transit over land. Antennas having cross-fed coaxial cables have been designed to provide for "break-down" or dismounting and folding of the antenna. Notably, the elements of such an antenna cannot be separated without severing the connections between the coaxial transmission lines which in turn destroys the electrical characteristics of each line. To provide for some degree of break-down, antennas of this type have been designed with support tubes having slip joint connections, allowing the support tubes to be separated and folded alongside one another (still interconnected by the transmission line). Breaking the antenna down in this manner leaves the coaxial cable exposed and the antenna in still relatively difficult to handle.
A second drawback of conventional co-linear stacked antennas is that they have limited adaptability. In some situations, it is desirable to extend or reduce the length of the antenna to increase its gain or reduce its size, respectively. The length of conventional antennas of this type is generally not modifiable.